\[
\begin{array}{l}
 (x + y) \cdot y' = 1 \\ 
 y' = \frac{1}{{x + y}} \\ 
 \frac{{dy}}{{dx}} = \frac{1}{{x + y}} \\ 
 \frac{{dx}}{{dy}} = \frac{{x + y}}{1} \\ 
 \frac{{dx}}{{dy}} = x + y \\ 
 \frac{{x' - x - y = 0}}{{}} \\ 
 \end{array}
\]